Transport of particulate solids during the production of oil or other fluid from a wellbore is a serious problem in the oil field.
The problem arises because in extracting oil from underground it is necessary to facilitate a flowpath for the oil to allow the oil to reach the wellbore. The oil is then produced by allowing it to travel up the wellbore to the surface of the ground.
Transported particulate solids sometimes clog the wellbore, thereby limiting or completely stopping oil production. Such solids represent a significant wear factor in well production equipment, including the pumps and seals used in the recovery and pumping process. Particles present in the pumped fluid sometimes cause excess friction and greatly increase wear on sensitive portions of the fluid handling and production equipment. Finally, these particulate solids must be separated from the oil to render the oil commercially useful, adding even more expense and effort to the processing of oil.
Undesirable paticulate flowback materials that are transported in fluids flowing to the wellbore are particularly pronounced in unconsolidated formations. By "undesirable", it is meant that the flowback of the particle is undesirable. In some cases the particles flowed back may be proppant, which is desirable when in place in the formation (its intended function), but is not desirable if it flows out of the formation and up the wellbore. When that occurs, proppant particle becomes an undesirable contaminant because in that instance it acts to reduce, not increase, the production of oil from the well in an efficient manner.
In general, unconsolidated formations are those that are less structured, and therefore, more easily facilitate the uninhibited flow of fine particles. Further, particulates sometimes are located in the near wellbore area for reasons that are not simply based upon natural flow to such areas. In some cases, the presence of particulates is attributed to well treatments performed by the well operator that place particulate solids into the formation or the near wellbore area. Examples of such treatments are fracturing and gravel packing.
Numerous different methods have been attempted in an effort to find a solution to the problem of the undesirable flow of particulates. What has been needed in the industry is a method, material, or procedure that will act to limit or eliminate flowback of particulate materials placed into the formation in a fracturing process. Until the time of this invention, there was no-satisfactory method of reducing or eliminating flowback.
One method employed in the past is a method of gradually releasing the fracturing pressure once the fracturing operation has been completed so that the fracture closure pressure of the formation rock acting against the proppant builds gradually. In this way, the method allows proppant the matrix to stabilize before fracturing fluid flowback and well production operates to carry significant quantities of the proppant out of the fractures and back to the wellbore.
Another method that has been employed in some instances to assist in reducing flowback of particulates is the use of so-called "resin-coated proppant", that is, particulate proppant materials having an adherent coating bonded to the outer surface of the proppant so that the proppant particles are bonded to each other. This process further reduces the magnitude of proppant flowback in some cases. However, there are significant limitations to the use of resin-coated proppant. For example, resin coated proppant is significantly more expensive than other proppant materials, which significantly limits it application to less economically viable wells.
Fracturing treatments may employ thousands or even millions of pounds of proppant in a single well or series of wells. Thus, the use of expensive, resin-coated proppants is generally limited by economics of well operation to only certain types of wells, or is sometimes limited to use in only the final stages of a fracturing treatment, sometimes known as the "tail" end of the fracturing job, or simply the "tail-in" of proppant near the end of the pumping job.
In unconsolidated formations, it is common to place a filtration bed of gravel in the near-wellbore area to present a physical barrier to the transport of unconsolidated formation fines with the production of wellbore fluids. Typically, such so-called "gravel packing operations" involve the pumping and placement of a quantity of gravel and/or sand having a mesh size between 10 and 60 U.S. Standard Sieve Series mesh into the unconsolidated formation adjacent to the wellbore. It is sometimes desirable to bind the gravel particles together to form a porous matrix for passage of formation fluids while facilitating the filtering out and retainment in the well of the bulk of the unconsolidated sand and/or fines transported to the near wellbore area by the formation fluids. The gravel particles may constitute a resin-coated gravel which is either pre-cured or can be cured by an overflush of a chemical binding agent once the gravel is in place. In some instances, various binding agents have been applied to the gravel particles to bind them together, forming a porous matrix.
Unfortunately, gravel packing is a costly and elaborate procedure that is to be avoided if possible. Further, some wellbores are not stable, and thus cannot be gravel packed. Further, gravel packing does not completely eliminate the production of fines particulates, and it is preferable to avoid the production of particulates without employing a gravel packing operation if possible. Gravel packing will not work in all instances.
Another recurring problem in pumping wellbore fluids is the enormous amounts of energy required to pump fluids containing large proppant concentrations at high rates for relatively long periods of time. Large amounts of energy are needed to overcome the great frictional forces between the proppant slurry and the interior of the tubular through which the slurry is being pumped. Above a certain threshold pressure, the fluid/proppant mixture cannot be pumped at all, because of the great frictional forces present at the liquid/tubular interface on the interior surface of the tubular or wellbore. The industry needs a viable solution to the problem of excess friction during pumping of proppant. Further, the industry needs a method or fluid that will inhibit production of particles, proppant and fines without substantially adversely effecting oil recovery from the wellbore.